I have 18. They all used to see regular play either from me personally, or from lending them out. By regular I mean twice a month or so. But, now that I'm focused on judging to try to get to level 2, and when I do play I am playing more modern, only my top 3 or so favorite decks see regular play and some haven't been played in months.
Just look at how many games you play a month and how often you want a deck to be played to be 'worth it'